Method for controlling the moisture of a web in machine direction on a coating machine

ABSTRACT

The invention relates to a method by means of which the machine-direction moisture of a web being coated can be controlled in an optimal manner that takes into account moisture content changes along the entire path of the coating and drying process. Advantageously, all the dryers the coater section are controlled in an integrated manner in order to obtain a controlledly processed product which is optimized in regard to energy consumption and product quality. Each process section and unit contributing to the drying of the web is identified by means of a mathematical submodel describing the specific evaporation rate in the respective process section/unit and, by chaining these submodels, a composite model is compiled for the entire process, whereby the composite model makes it possible to manage the drying operation in the process so that the individual units are controlled as a portion of the overall process.

PRIORITY CLAIM

This is a national stage application under 35 U.S.C. §371 ofinternational stage PCT application No. PCT/FI00/00166, filed on Mar. 2,2000. Priority is claimed under 35 U.S.C. §119(a) and 35 U.S.C. §365(b)from Finnish Patent Application No. 990474, which was filed in Finlandon Mar. 4, 1999, and from which priority was properly claimed in theaforementioned international stage application.

FIELD OF THE INVENTION

The present invention relates to a method based on a novel control andsteering strategy for use in the drying process of a paper web orsimilar coated web material such as a board in coater sections in whichthe web to be coated is passed via a coater station including at leastone applicator apparatus and dryers.

BACKGROUND OF THE INVENTION

In the coating of a web of paper or board, the surface of the web isfirst coated with a furnish containing coating pigments slurried inwater. After the application and smoothing of the coating mix, thecoating applied to the web surface as well as the underlying base webmust be dried to a sufficiently low moisture for final use or furtherprocessing. Hence, a major portion of the energy consumed in theproduction of coated paper grades is lost in drying the web during thedifferent steps of postprocessing, which means that energy management indrying is an extremely vital factor contributing to the profitability ofproduction. Correct drying technique also affects the quality of theproduced paper grade. Another parameter highly pertinent to the qualityof produced paper is the control of the machine-direction moistureprofile, that is, the moisture of the base paper, which must be kept ata constant level during the run. The web moisture content affectsparticularly the paper web behavior in calendering and printing. Asmodern production lines are equipped with on-line calendering, whereinthe coated web is passed directly to a calender, the moisture profile ofthe running web has an insufficient time to reach a uniform equilibriumstate prior to calendering, a situation which is in contrast to thatattainable in the traditional off-line calendering, wherein the coatedweb was stored in a machine reel prior to subsequent calendering.Correspondingly, the transport chain of paper from the mill to printinghouses and other users has been speeded up, whereby the moisture even inuncalendered paper does not necessarily have enough time to stabilizeand reach a sufficiently low level prior to printing. In coating, theweb moisture content affects the penetration of water into the base webduring the application of the coating mix and, as a result, the changeof coating solids content after coating. As variations in the solidscontent of the coating are reflected in plural parameters in theapplication process, it is important to keep the web moisture duringapplication and drying accurately within proper limit values in order toattain a uniform and desired final quality of the product.

Conventionally, a coated web is dried immediately after the applicationof coating using noncontacting dryers, which step may be followed whennecessary by cylinder dryers and other dryers of the contacting type.The moisture content of the running web is measured at multiple pointsalong the web travel in the coater apparatus and, on the basis of themeasurement data, the drying effect of each dryer is individuallyadjusted so as to attain a proper web moisture over the cross-machinewidth at the respective measurement point as well as an average moisturecontent that stays between given limits during a run, the latterrequirement meaning that the machine-direction moisture profile iscontrolled to a given set value. The overall drying capacity is adjustedto a suitable basic level based on test runs and data accumulated from along-term experience in the art, and the individual dryer effects arethen fine-tuned during the run on the basis of measurement data eitherautomatically or manually. Conventionally, one of the dryers or onedryer group is selected to perform as the controller of the finalmoisture level, whereby the heating power input to the selected dryergroup(s) is adjusted by means of a feedback signal obtained from themeasurement system. In this arrangement, the other dryers are drivenunder manual control. Such a control scheme responds very tardy andcompensation for the slow response of dryer control is difficult toimplement in situations requiring a fast change of dryer effect levels.Furthermore, the web temperature prior to the coater apparatus must bekept sufficiently low to avoid floccing of the coating mix beingapplied. Hence, proper control of the drying effect is importantparticularly in the final stage of the dryer section prior to thesubsequent coating step. The web temperature also affects the finalquality of the coated web.

Particularly in situations of changing running conditions or whenstarting up the machine, known in the art as the run-up, the elevationof the dryer drying effect levels to correct values and adjustment ofthe same to proper run-time levels requires excellent skills from thepersonnel operating the machine. However, carrying out the procedure ofsetting the dryer evaporation effect levels in the coater section tocorrect values under run-up or changing process conditions takes time,during which the produced paper or board falls short of the specifiedquality requirements thus necessitating dumping of the web into thepulper. Hence, it is advantageous to minimize the durations of run-upand process value change times in order to achieve improved productionefficiency at the machine. The above control scheme is also extremelyclumsy in the optimization of drying energy consumption inasmuch itrelies on the control of each dryer unit separately, whereby the mutualevaporation effect ratios between the dryer units are difficult to alterin an uncomplicated manner. Furthermore, a failure in one or a greaternumber of the dryer units is difficult to compensate for, because theprocess is designed for operation with all the dryer units beingfunctional.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method suited forcontrolling the machine direction moisture profile of a web to be coatedin a manner optimized to respond to any moisture changes throughout theentire coating/drying process. In practice this approach means theapplication of a comprehensive control scheme covering all the dryerunits of a coater section in an integrated manner in regard to energyconsumption and product quality in order to attain an optimal endresult.

The goal of the invention is achieved by way of forming a mathematicalsubmodel of specific moisture evaporation rate for each process sectionand device contributing to the web drying process and then chaining thethus obtained individual submodels so as to form a composite model ofthe overall process, the model being suited for managing the dryingphenomena during the entire process so that each individual unit of theequipment layout is controlled as a part of the overall process.

The invention offers significant benefits.

By virtue of the model according to the invention, it is possible todirectly compute the moisture content of the web at the outgoing side ofeach dryer, provided that the specific evaporation rate at the dryer andthe web moisture at the ingoing side are known. After the chaining ofthe individual submodels, the web moisture content can be computed atdifferent points along the coater section, the most important parametervalue obviously being the final moisture content of the web. With thehelp of the model, the dryer effects may be adjusted according to theindividual properties so that the characteristics of different types ofdryers are optimally taken into account. Since infrared dryers feature aquick response, they may be used, e.g., during run-up for controllingthe overall effect of the dryer group, thus allowing the evaporationeffect levels of other dryers to be elevated in a more relaxed manner totheir steady-state values during the normal run by way of compensatingfor the delay of dryer warm-up with the help of delay terms adapted intothe model. The use of delay terms also makes it possible to manageactual process response delays.

Since the invention provides a control scheme for the overall process,it allows the evaporation effects of the dryer units to be dividedtherebetween in a desired manner and, particularly in the case offailure in one dryer, the drying effect lost thereby may be compensatedfor by the other dryer units thus permitting operation of the coatersection uninterrupted by a servicing shutdown. Equally, as the initialmoisture content of the web as well as the amount of moisture addedthereto by the applied coating are known, the model gives tools forcomputing an estimate for the web moisture at different points along theprocess and, particularly, prior to upwinding. In fact, the model allowsthe web final moisture content to be computed so accurately thatproduction may be continued controlled by the model even when themoisture measurement devices are down.

The overall performance offered by the invention gives a faster and moreaccurate control result than that available by way of manual controlcombined with feedback loops controlling the individual drying units.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims. It should be further understood that thedrawings are not necessarily drawn to scale and that, unless otherwiseindicated, they are merely intended to conceptually illustrate thestructures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be examined in greater detail bymaking reference to the appended drawings in which

FIG. 1 shows a coater section or a portion thereof comprising one coaterstation and dryers;

FIG. 2 shows a coater section or a portion thereof comprising two coaterstations, each equipped with separate dryers;

FIG. 3 shows a schematic plot of changing web speed in the coatersection;

FIG. 4 shows a schematic plot of the control of the dryer effect at achange in the web speed; and

FIG. 5 shows block diagram of the present control method.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENT

Referring to FIG. 1, the diagram shown therein schematically illustratesa layout comprising one coater station 1 with dryers 2-6 connectedthereto, whereby the functions related to the drying effect control ofthe dryers 2-6 are illustrated as a block diagram. In the downstreamtravel direction of a web 8, the first unit is a coater station 1serving to apply coating or other web treatment substance such assurface size to at least one side of the web. As the type of coaterstation used is irrelevant to the implementation of the invention, thecoater may be any suitable applicator apparatus such as a short-dwellcoater, film-transfer coater, blade coater or spray coater. The coaterstation may be used for applying the coating to one side of the web 6only as is customary, or to both sides, as required. Obviously, theconstruction of dryers 2-6 is dependent on whether two-sided orone-sided coating is performed in a single coater station, butnevertheless the function of any one of the dryers may be modeled in thesame fashion in accordance with the invention.

Next downstream to the coater station 1 are located first an infraredradiant dryer 2, then three air-impingement dryers 3 to 5 and finally adryer cylinder group 6 comprising a plurality of dryer cylinders 7. Onthe dryer cylinder group 6, the web 8 is dried to a moisture suitablefor final calendering and next the web 8 is passed via a moisturecontent gauge 9 to the upwinder 10.

The process is controlled by means of a computer. The actualimplementation of the computer may comprise a module running under thesoftware of the coater section control computer, or a separatelyallocated computer or microprocessor serving the moisture control taskalone or a physically distributed software and database package. Thecontrol system contains an evaporation rate submodel for each one of thedryers and a composite evaporation rate model compiled from thesesubmodels. Additionally, the data base 11 of the control system servesto store the process status data, that is, the real-time status of boththe coater section and the model obtained by way of measurement ordirectly from the computational data submitted by control system of thecoater section. The status data includes such parameter values as thecoater section status comprising the applied coat weight, solids contentthereof and other similar factors, dryer evaporation effect levels, thefinal moisture content after the dryer units and the web speed asmeasured at the upwinder 10.

In FIG. 2 is shown a system comprising two coater stations, eachequipped with separate dryers. While the exemplifying embodiment isdescribed below for the two last coater stations of a system, a completelayout may include a plurality of such subsystems formed by a coaterstation with dryers. Then, each subsystem may be described by means ofan individual evaporation rate submodel, or more advantageously, asingle model is compiled for the entire coater section, thus achieving asimpler control scheme for managing the overall process. In certaincases, particularly the web moisture prior to its entry into asuccessive coating step may exceed the final moisture content of theweb, which means that the average moisture content of the web whenpassing through the coater section is higher than when the web reachesthe upwinder. This kind of situation requires more drying effect afterthe last coater station than on the preceding stations, which is easy toarrange by virtue of the method according to the invention so that thecomputed or measured web moisture content value from the output of anext successive subsystem is fed back to the evaporation effectcomputation of the preceding subsystem. Conventionally, the moisture ofweb passed from the papermaking machine to the coater is in the range ofabout 1.5-4%, while the moisture of a treated web in the order of 4-6%.The moisture content values of the web may vary widely during thedifferent phases of web treatment, and also the final and initialmoisture contents of the web may vary according to the grade beingmanufactured. If desired, the initial moisture content of the web may becomputed on the basis of the amount of evaporation during a run, thisvalue being obtained from the model, and the final moisture content ofthe web, this value being measured prior to the upwinder.

Accordingly, the method according to the invention aims to provide anaccurate overall control of the machine direction moisture profile ofthe web along its entire downstream travel through the coater section inall production situations and, particularly, during the dynamictransition phase toward the steady-state production run condition, thatis, during machine run-up and when changes occur in the machine speed orthe coating process parameters. The present method is capable ofcontrolling a plurality of coater section dryers simultaneously so thatthe target value of web moisture is attained optimally. In the novelapproach according to the invention, each drying unit is formulated withthe help of a mathematical submodel describing the specific evaporationbehavior of the unit, whereupon the submodel is utilized in thecomprehensive control strategy for computing the unit-specific setvalues. The thus formulated specific evaporation rate models are used ina chained manner for modeling the overall process, complemented withcertain measurement results obtained from the process. The parameters ofthe mathematical modeling equations may be updated on either per unit orper operating point basis using either off-line or on-line techniques.The thus obtained computational model can be brought to match exactlywith the operation of the coater section for different kinds ofmanufactured product grades and varying process conditions.

The method can be applied to both so-called off-machine and on-machinecoater sections, and it is capable of performing dryer control functionsunder a normal steady-state production run situation as well as duringdynamic transition phases toward a normal production run state. In thecontext of the present invention, a normal steady-state production runsituation is understood to refer to a condition in which no changesoccur in the machine speed or, if changes do occur, they are of a typethat will not be reflected in the product quality. Such change andtransition situation(s) is/are represented by changes in machine speedand start-up of section operation. The measurements values of theprocess quality monitoring system and other values such as the webmoisture, basis weight, coat weight, coat solids content and webtemperature sensor signals obtained from the coater section controlsystem serve as the input signals of the method. The measurement sensorsof the process quality monitoring system may be located either after thelast dryer unit in each coater station and preceding the upwinder,whereby the measurement system represents a comprehensive implementationor a portion of the so-called intermediate points of moisturemeasurement can be omitted, whereby the method may use the web moistureestimates which are computed from the evaporation model and bear anaccurate relationship with the actual situation along the web travel,particularly when the parameters of the modeling equation are updated inreal time.

Based on the mathematical models, the present method computes thespecific evaporation rate, e.g., as kgH₂O/m²/h for each dryer or processunit contributing to the drying process. The computations take intoaccount the coater stations, infrared radiant dryers, air-impingementdryers, cylinder dryers and other dryers associated with the coatersection, as well as the open draws between the dryer units. Open drawsform an important part of the modeling task and must be included in thecomposite model, because moisture evaporation also takes place on theseportions of the web travel from the hot web exiting the dryers.

On a coater station, the coating applied to the surface of the webcarries along a certain amount of excess water that must be removed onthe dryers. When the initial moisture content of the web, as well as theamount of applied coating and the moisture content of the coating areknown, it is possible on the basis of the web speed to compute therequired overall evaporation effect and to divide it between thedifferent dryers. The goal is to control the so-called intermediatemoisture of the web after each coater station, as well the finalmoisture of the finished product to desired target values by means ofsteering the coater section dryers as an integrated system. The specificevaporation computation utilizes measurement data gathered on webmoisture, temperature, speed and on the ambient air humidity. With thehelp of the specific evaporation models, it is possible to compute anestimate for the moisture of the web leaving any dryer. Similarly, it ispossible to compute the change in web temperature within each processunit and the exit temperature of the web at the outgoing side of eachunit. A chained composite model for the entire system is obtained bycombining the mathematical submodel equations that describe the behaviorof the dryers and the open draws. Herein, the values of the web moistureand temperature computed for the outgoing side of a preceding dryer areused as the input values for the next dryer, that is, representing themoisture and temperature values of the entering web.

According to the method, the web intermediate moisture after each coaterstation and the final moisture content of the finished product at theupwinder are controlled by means of specific evaporation submodelsdeveloped for the dryers of the coater section. With the help of thesesubmodels it is possible to compute such set values of adjustment andcontrol variables for each modeled unit that bring about the desiredvalues of web intermediate and final moisture contents. The sameapproach also is used to manage a machine speed change situation. Thecontrol actions are carried out with the help of both closed-loopfeedback circuits and feedforward circuits. Moisture measurement signalsobtained from the process quality monitoring system are taken to thefeedback circuit that adjusts the set values of one or more dryer unitsin the coater section. The feedforward circuit, which is employed tomanage the dynamic transition states of machine speed change, uses setvalue estimates which are computed from the mathematical submodels ofthe specific evaporation rates for the final condition of the machinespeed change state. This description, however, omits the details of theactual modeling techniques used inasmuch those skilled in the art haveno difficulty in finding the needed mathematical tools in theliterature.

The first step in the method according to the invention is to computethe specific evaporation rates for the different units of the productionline. The specific evaporation rates as kgH₂O/m²/h are computed for theseparate dryers of the coater section using the computational facilitiesof the automation system of the production line or of a separatecomputing unit intimately communicating therewith. The mathematicalsubmodels of the coater section dryers are developed separately for thecoater stations, infrared radiant dryers, air-impingement dryers andcylinder dryers and other dryers possibly cooperating with the coatersection, and for the open draws. The mathematical submodels take intoaccount the contribution of the characteristic control parameters ofeach unit and the effect of process variables on the overall specificevaporation rate. Such contributing variables include the web speed, theweb initial moisture and temperature, the web basis weight, the coatweight, the solids content and composition of applied coating, airhumidity, the lineal effect (kW/m) of the infrared radiant dryer, thetemperature and flow rate of impinging air blown in the air-impingementdryer, and the steam pressure and flow rate in cylinder dryers. As anoutcome of the computation, the submodels give the specific evaporationrate for each dryer, the web moisture at the outgoing side of the dryerand the web temperature at a given point of interest when properlyselected control variables are used in the equations.

With the help of data obtained from the process quality monitoringsystem, the characteristic parameters of the evaporation rate submodelsmay be corrected, e.g., as per paper grade and system operating status.In this fashion, the composite model can be tuned to accurately matchthe actual operating status and the behavior of the coater section to becontrolled. To this end, the estimate obtained from the model for theweb moisture at a given point of the web travel, e.g., prior toupwinding, is compared with the actual moisture data obtained from theweb measurement sensors. On the basis of this comparison, an error termis computed for the model that is then used in the correctioncomputation for the model parameters. The correction computation may becarried out as either an off-line task within the automation system ofthe production line or other computing system connected thereto oralternatively, directly as an on-line task in the automation system,using appropriate computing routines such as the least squares method,for instance, or equivalent recursive algorithms. For this purpose, thedryers are controlled according to a specific strategy so that all thedryers are set to a constant evaporation effect state, with theexception of the one for which the equation parameters of the submodelare to be analyzed. During the parameter value update operation, thecontrol signals of the dryer being analyzed are appropriately varied inaccordance with the parameter identification technique used, e.g., byway of imposing stepwise changes in the set value or superimposing aPRBS (pseudo-random binary signal) on the set value output signals inorder to cause a sufficient amount of changes in the system beinganalyzed so that the computational algorithm of the parameteridentification technique will converge. The thus obtained parametervalues of the modeling equations as per paper grade and processoperating point can be stored in a separate database or in thegrade-specific production control files of the process automationsystem.

According to the invention, moisture control along the downstream travelof the web takes place as follows. In the method described herein, amodel-based web moisture controller computes from the actual measurementsignal of the web moisture and the target value of the web moisture acontrol signal, whereby the computational process utilizes a compositemodel compiled from the mathematical submodels of the individual dryers.The computation takes into account the specific evaporation rates of thedryers and the prevailing manufacturing process conditions. With thehelp of the submodels, such set values of adjustment and controlvariables are computed for each dryer separately that are required toattain the desired intermediate and final values of web moisture. Duringdynamic changes of machine speed, the control algorithm computes theneed for effect change in the dryers according to the change in webspeed.

In a normal steady-state production run situation involving no change inweb speed, a feedback-type control scheme is used, whereby the modelinput signals formed by the web moisture set value and the actual webmoisture measurement information are processed into a feedback signal ofmoisture error, on the basis of which signal the control algorithm thenperforms required changes to an extent defined by the system operator inthe drying effects of dryers selected to be controlled by the controlcomputer. While all the dryers may be set to be controlled by a computeror, respectively, set for manual control, in the spirit of the inventionthe drying effect of at least one dryer must be steerable by means of amodel running on a computer. Herein, as shown in FIG. 2, either theintermediate point moisture sensor 12 or the process quality monitoringsystem sensor 9 preceding the upwinder 10 give the actual web moisturecontent value that is compared by the control program with the setvalue. On the basis of the difference between the set value and actuallymeasured web moisture, the system computes the respective change of theoverall moisture (ΔH₂O) that should be accomplished by means of thedryers selected to be steered by the control computer. If the moisturedifference signal has a positive sign, the specific evaporation ratemust be increased. Respectively, a negative sign indicates a need forreduced specific evaporation rate. The overall value of requiredmoisture change (A H₂O) is divided between the dryers (i=1−N) selectedto be steered under computer control using such proportional percentageweight factors (0-100%) that the sum of the weighting factors always is100%. Obviously, other weighting strategies are also possible in thedivision of moisture change, that is, to implement the required changein the distribution of the drying effect between the dryers. Forinstance, the weighting factors may be selected to be proportional tothe available evaporation rate capacities on the modeled dryers or tothe desired moisture values at the intermediate points. In this kind ofproportional division, each of the selected dryers is allocated tohandle so much of the overall moisture difference control task as isindicated by its weighting factor. The specific evaporation rate modelsare then used for computing the required changes in the set values ofcontrol signals given to each one of the selected dryers. Aftercomputation, the new set values are transmitted to the unit controllersthat implement the changes in the set values.

In FIG. 3 is shown a situation involving a change in the machine speed.In this case, the control scheme relies on a feedforward circuit. Toperform a change in the machine speed from point A of the diagram topoint C, the procedure goes as follows. The new set values required atpoint C for the dryers of the coater section are computed at point Ausing the submodel equations so that the correction to be made in theset values due to the machine speed change are taken into account. Thenew set values can be transmitted to the unit controllers eitherimmediately at the start of the machine speed change (point A) orincrementally over the entire duration of the machine speed change phaseas shown in the diagram of FIG. 4. The choice of either control strategyis dictated by the amount of machine speed change (ΔL), duration of thechange (ΔT) and the dynamic behavior of the selected dryer. Duringmachine run-up, the control strategy can be, e.g., as shown in theright-hand plot of FIG. 3. The new set values required for the unitcontrollers at either point B′ or point C′ are computed at point A′ withthe help of the modeling equations. If the acceleration of the machinetakes place via an intermediate point B′, the corresponding set valuesfor the target speed at point C′ can be transmitted at either points A′,B′ or in an incrementally stepwise manner (see FIG. 4). For controllingsuch fast-response dryers as infrared radiant dryers, a desired numberof incremental point values may be computed on the basis of the setvalue start and end points, whereby the incremental values are activatedwhen the machine attains a speed corresponding a given set value. On theother hand, if the slow dynamic response at, e.g., air-impingementdryers and cylinder dryers (characterized by a delay time t) is takeninto account, the set values corresponding to point C′ for the selectedunits may be transmitted already at point (A′−t) or the delayed responsemay compensated for in the incremental control. The intermediate pointB′ is most conventionally used, e.g., for shutting down the coaterstations. Herein, depending on the time span (ΔT′), it is possible tocompute also for point B′ the set values of the unit controllers thatare then used as the input values in the computation of variable setvalues for transition toward state C′. The method can also handlesituations in which full-width moisture measurement information obtainedfrom the product quality monitoring system or partial-width moistureprofile measurements are utilized during machine speed changes or systemrun-up. In a partial-width moisture profile measurement, the moisturesensor of the product quality monitoring system may be of a so-calledfixed type (nontraversing) or the sensor may be arranged to perform atraversing movement, that is, in the cross-machine direction, only for apartial width of the web covering the web by 0.5-1.0 m, for instance. Inthis case the arrival of a new, reliable moisture measurement valuealways triggers a corrective action performed with the help of themodeling equations or other correction computation on the estimates ofset values transmitted to the dryers.

In FIG. 5 is shown the above-described control strategy in a slightlydifferent diagram. The control scheme illustrated in this block diagramis equivalent to that shown in FIGS. 1 and 2. The left-hand part of thediagram depicts the determination of the moisture difference value. Tothis end, the first step is to measure the web speed, whereupon it ispossible to employ the data on coat thickness, moisture, base web basisweigh and coat solids content in the determination of the web moistureor change of moisture ΔH₂O at the ingoing side of a dryer. When themoisture set value is summed with the measured value, or the actualvalue, of web moisture, the result is the difference ΔM between the setvalue and the actual value, wherefrom is possible to compute the neededdrying effect change ΔH₂O that must be summed with other possiblechanges caused by process deviations. On the basis of the thus obtainedneed for drying effect change, the needed drying effects and new dryerset values are computed with the help of the composite model of theprocess and the actual values of process conditions and process status.After computation, the new set values are transmitted to the dryers.

The method according to the invention can be applied to all kinds ofpaper/board coating techniques and equipment in which the surface of abase web is coated with a liquid-based that is dried on at least onedryer. Generally, however, the layout comprises plural dryers and, infact, the benefits of the invention will be the greater the morecomplicated the coater section is.

Thus, while there have shown and described and pointed out fundamentalnovel features of the invention as applied to a preferred embodimentthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices illustrated, and intheir operation, may be made by those skilled in the art withoutdeparting from the spirit of the invention. For example, it is expresslyintended that all combinations of those elements which performsubstantially the same function in substantially the same way to achievethe same results are within the scope of the invention. Moreover, itshould be recognized that structures and/or elements shown and/ordescribed in connection with any disclosed form or embodiment of theinvention may be incorporated in any other disclosed or described orsuggested form or embodiment as a general matter of design choice. It isthe intention, therefore, to be limited only as indicated by the scopeof the claims appended hereto.

1. A method for controlling a drying effect of an equipment layout usedin making a coated web of paper or board, the equipment layoutcomprising at least one coater unit and a plurality of drying units, themethod comprising: applying a liquid-containing coating to a surface ofa web in the at least one coater unit; drying the web coated with thecoating in the plurality of drying units by evaporating the liquid fromthe coated web until a moisture content of the web reaches a desiredfinal moisture value; compiling, for each drying unit wherein moistureis evaporated from the web, an evaporation rate submodel suited forcomputing an amount of liquid removed by the respective drying unit,wherein at least one specific evaporation rate submodel is linked into acomposite evaporation rate model; determining a needed overallevaporation effect to be performed by the equipment layout to achievethe desired final moisture value; determining, by employing thecomposite evaporation rate model, a needed moisture evaporation effectfor each drying unit of the equipment layout having an evaporation ratesubmodel linked into the composite rate model to achieve the neededoverall evaporation effect; and controlling the moisture evaporationrate for each drying unit of the equipment layout having an evaporationrate submodel linked into the composite rate model, the moistureevaporation rate being controlled in accordance with the determinedneeded moisture evaporation effect without measuring an actual amount ofliquid removed by any individual drying unit of the equipment layoutwhile such drying unit is drying the web.
 2. The method according toclaim 1, further comprising: controlling an evaporation effect of onedrying unit of the equipment layout having an evaporation rate submodellinked into the composite rate model with the composite evaporation ratemodel; and setting the evaporation effect of every other drying unit ofthe equipment layout to a fixed value.
 3. The method according to claim2, wherein a control signal to the controlled drying unit is changed inat least one of a stepwise manner and a superimposition of apseudo-random binary signal (PRBS) on at least one set value.
 4. Themethod according to claim 1, further comprising: measuring a finalmoisture content of the web attained after drying the web with thedrying units of the equipment layout; comparing the measured finalmoisture content with the desired final moisture value; and controllingthe moisture evaporation rate for at least one drying unit of theequipment layout with the composite evaporation rate model based uponthe comparison of the measured final moisture content with the desiredfinal moisture value.
 5. The method according to claim 1, wherein anyneeded change in the overall moisture evaporation effect is allocatedamong drying units for which the evaporation rate is controlled usingthe composite evaporation rate model proportionately in ratiosdetermined by predetermined weighting factors.
 6. The method accordingto claim 1, further comprising: measuring an initial moisture content ofthe web prior to entering a first of the at least one coater unit;determining the amount of liquid applied to the web in said applyingstep; and controlling, using the composite evaporation rate model, themeasured initial moisture content and the determined amount of liquidapplied to the web, an evaporation rate of at least one controllabledrying unit of the equipment layout.
 7. The method according to claim 1,wherein an output value obtained from the evaporation rate submodel of adrying unit of the equipment layout is used as an input value in theevaporation rate submodel of a next successive drying unit of theequipment layout.
 8. The method according to claim 1, wherein an outputvalue obtained from the evaporation rate submodel of a unit of theequipment layout is used as input value in the evaporation rate submodelof a preceding unit of the equipment layout.
 9. The method according toclaim 1, wherein the equipment layout comprises a plurality ofsubsystems, each subsystem comprising at least one coater unit and atleast one dryer unit, and wherein an output value obtained from theevaporation rate submodel of a subsystem is used as input value in theevaporation rate submodel of a preceding subsystem.
 10. The methodaccording to claim 1, wherein the equipment layout comprises a pluralityof subsystems, each subsystem comprising at least one coater unit and atleast one dryer unit, and each subsystem having a respective evaporationrate submodel, and wherein the subsystem evaporation rate submodelsinteract to produce the needed overall moisture effect of the equipmentlayout.
 11. A method for controlling the drying of a coated web of paperand/or board in a web apparatus comprised of at least one coater, atleast one dryer, and at least one open draw after at least one dryer,the method comprising the step of: using a composite drying effect modelof the overall drying effect of the web apparatus to control the dryingof the coated web, said composite drying effect model being comprised ofa plurality of drying effect submodels linked together; wherein each ofthe at least one coater, the at least one dryer, and the at least oneopen draw has a drying effect submodel of its drying effect on thecoated web included in the plurality of drying effect submodelscomprising the composite drying effect model; wherein an output of eachsubmodel comprises an evaporation rate of the component represented bysaid submodel, a moisture value of the web output from the componentrepresented by said submodel, a change in web temperature within thecomponent represented by said submodel, and/or a temperature of the weboutput by the component represented by said submodel; and wherein anoutput of at least one submodel in the composite drying effect modelcomprises an input of a submodel following said at least one submodel inthe composite drying effect model.
 12. The method according to claim 11,wherein the step of using a composite drying effect model of the overalldrying effect of the web apparatus to control the drying of the coatedweb comprises the sub-step of: complementing the composite drying effectmodel with measurement results from the web apparatus.
 13. The methodaccording to claim 11, wherein submodels for individual components arecombined together to form composite submodels.
 14. The method accordingto claim 13, wherein the web apparatus comprises at least two coatingstations, each of the at least two coating stations comprising at leastone coater and at least one dryer, and wherein submodels for individualcomponents in each of the at least two coating stations of the webapparatus are combined together to form a composite submodel.
 15. Themethod according to claim 14, wherein the step of using a compositedrying effect model of the overall drying effect of the web apparatus tocontrol the drying of the coated web comprises the sub-step of: using afeedback system with a coating station composite submodel to adjust setvalues of one or more of the at least one dryer in the coating stationusing moisture measurements from the output web of the coating section.16. The method according to claim 14, wherein the step of using acomposite drying effect model of the overall drying effect of the webapparatus to control the drying of the coated web comprises the sub-stepof: using a feedforward system with a coating station composite submodelto manage dynamic transition states during web speed changes.
 17. Themethod according to claim 13, wherein an output of a composite submodelis fed back as input to a previous composite submodel in the chainforming the composite drying effect model.
 18. The method according toclaim 11, further comprising the step of: using the composite dryingeffect model to calculate an initial moisture content and/orintermediate moisture content of the web using a measurement of thefinal moisture content of the web.
 19. The method according to claim 11,further comprising the step of: using the composite drying effect modelto calculate a final moisture content of the web using a measurement ofan initial moisture content and/or intermediate moisture content of theweb.
 20. The method according to claim 11, wherein each submodel is of adrying effect on the coated web by a component comprising the webapparatus, and wherein the step of using a composite drying effect modelof the overall drying effect of the web apparatus to control the dryingof the coated web comprises the sub-step of: taking into account, foreach submodel, the contribution of characteristic control parameters ofits corresponding component and the effect of process variables on theoverall drying effect of the web apparatus.